Nitride semiconductors represented by gallium nitride (GaN), aluminum nitride (AlN), and the like have a higher electron mobility and a wider band gap than silicon, and are expected to be used for implementing an electronic device taking advantage of those features, such as a very-high-speed transistor or a very-low-loss switching device. Because of such excellent material properties, devices employing nitride semiconductors are expected to surpass the devices based on semiconductor silicon, which are mainly used at present, in material-property limit.
As substrates for the epitaxial growth of such a nitride semiconductor thereon, use has hitherto been made of sapphire, silicon (Si), silicon carbide (SiC), zinc oxide (ZnO), and the like. Of these substrates, single-crystal silicon substrates are suitable because the substrates having better crystallinity, a larger area, and a higher purity can be produced at lower cost as compared with the other substrates. In addition, when a single-crystal silicon substrate is used, the device steps currently in use can be employed, without any modification, as subsequent device steps. Use of single-crystal silicon substrates is hence superior also in development cost, and there is a desire for practical use thereof.
However, a comparison between the coefficient of room-temperature expansion of single-crystal silicon substrates and that of nitride semiconductors shows that the coefficient of room-temperature expansion of the nitride semiconductors have a value approximately two times the value for the single-crystal silicon substrates. Because of this, when epitaxial growth is conducted on a single-crystal silicon substrate using a method having a relatively high growth temperature, such as organometallic vapor phase epitaxy, the nitride semiconductor layer undergoes tensile stress and cracks when the substrate temperature is lowered to room temperature after the growth. Furthermore, since the difference in crystal lattice constant between silicon and the nitride semiconductors is as large as 10% or more, there is a problem that the difference causes crystal defects, etc.
In order to overcome those problems, there has been proposed a semiconductor device which includes a single-crystal silicon substrate, a buffer layer which is formed on the substrate and has a multilayer structure, and a semiconductor device formation region formed on the buffer layer (e.g., JP-A-2003-59948).
However, there is a problem that formation of a thick buffer layer and a thick nitride semiconductor layer on a single-crystal silicon substrate by epitaxial growth results in enhanced warpage of the single-crystal silicon substrate. To deposit a buffer layer or nitride semiconductor layer in an increased thickness, on the other hand, has an advantage that the increase in thickness leads to an improvement in the crystallinity of the layers themselves.
For the purpose of reducing warpage, there has been proposed a compound semiconductor substrate which includes a single-crystal silicon substrate and, formed thereon, a buffer layer including: multilayered buffer regions each composed of a plurality of superposed thin unit layers differing in materials and a thick single-layer buffer region having evenness of material and disposed between the multilayered buffer regions (e.g., JP-A-2008-205117).
In general, these multilayered buffer regions are constituted of a combination of two or more materials differing in lattice constant, and the multilayered buffer regions are constituted of either the same nitrides of Group III elements or compounds thereof as device formation regions. InN, GaN, and AlN, which are typical Group III element nitrides, have the following relationship concerning the greatness of lattice constant: InN>GaN>AlN. The lattice constant of a mixed crystal of these (for example, AlxInyGa1-x-yN: in which (x+y)≧1) is determined by the proportions of the components. The higher the proportion of indium, the larger the lattice constant; and the higher the proportion of aluminum, the smaller the lattice constant. When use of GaN in device formation regions is taken into account, it is preferred from the standpoint of conditions for the growth thereof that a mixed crystal of nitrides mainly including GaN and a mixed crystal of nitrides mainly including AlN should be used in the multilayered buffer regions.
However, when the multilayered buffer regions described in JP-A-2008-205117 are formed in such a manner that a mixed crystal mainly including AlN is grown, without reducing crystal quality, under conditions close to optimal growth conditions for a mixed crystal mainly including GaN, then it is necessary to reduce growth rate because the optimal growth conditions for the mixed crystal mainly including AlN include a higher temperature than the optimal growth conditions for the mixed crystal mainly including GaN. Consequently, in the production of a nitride semiconductor substrate employing the above-mentioned multilayered buffer regions, it is desirable for heightening productivity without lowering quality that the proportion of the AlN-based mixed crystal in the multilayered buffer regions should be reduced. However, there is a problem that when the thickness per layer in a multilayered buffer region of the conventional structure is merely reduced in order to lower the proportion of the AlN-based mixed crystal, then there arises a problem that it is difficult to control cracking and warpage.